With equipment requiring for example, maintenance of mechanical moving parts, or coordinated (alternate) operation between an operator and a mechanical moving part, measures to ensure operator safety are extremely important.
In situations for example, where an operator approaches a danger region wherein it is possible to touch a mechanical moving part, conventional methods of ensuring safety when the mechanical moving part is in a movable condition, involve first shutting off the motor power supply in the case of motor driven mechanical moving parts, or shutting off the pressure supply in the case of pressure driven mechanical moving parts. The operator then verifies that the mechanical moving part has stopped, before approaching the mechanical moving part.
In practice however, there is the case wherein the operator approaches the mechanical moving part with the judgment that it will soon stop although it has not completely stopped. There is also the case wherein the operator mistakenly approaches while the mechanical moving part is moving. Furthermore, there is the case wherein the mechanical moving part which normally stops after a predetermined time interval, does not stop within the normal period due to a fault in the mechanical side (for example a fault in the braking).
It is therefore necessary to implement some type of safety measure. Conventional methods involve surrounding the mechanical moving part with a safety enclosure. There is also the case which adopt an arrangement for shutting off the drive power source to the mechanical moving part when the door to the safety enclosure is opened.
However, even though the drive power source to the mechanical moving part may for example be shut off by opening the door, since the mechanical moving part cannot stop instantaneously, there is the possibility of the operator approaching the mechanical moving part inside the safety enclosure before it has completely stopped.
Here an interlock system can be considered with a door lock incorporating a solenoid, provided in the door of the safety enclosure, so that the lock is released when the solenoid is excited. With this interlock system, the solenoid is excited when the mechanical moving part inside the safety enclosure stops, thereby releasing the lock only after the mechanical moving part has stopped.
Such an interlock system, necessitates a sensor which detects the stopping of the mechanical moving part, and stops the solenoid. An example, of such a sensor for detecting the stopping of the mechanical moving part and exciting the solenoid is shown in FIG. 1.
In FIG. 1, a rotation body 3 is provided by way of a speed reducing mechanism 2 on a motor 1 which is used to drive a mechanical moving part. A rotation disc 4 having a plurality of apertures 4a around a periphery thereof, is attached to the rotation body 3. A light emitting element 5 and a light receiving element 6 of a photo-interrupter are positioned opposite each other on either side of the rotation disc 4.
When the motor 1 is rotating, the light beam from the light emitting element 5 is received intermittently by the light receiving element 6 through the apertures 4a, depending on the rotation of the rotation disc 4. An alternating current signal as shown in FIG. 1 is therefore output from the light receiving element 6. This is amplified by an AC amplifier 7 and rectified by a rectifying circuit 8 to produce a direct current output signal. When the motor 1 is not rotating however, the light beam from the light emitting element 5 is either shut off by the rotation disc 4 so as not to reach the light receiving element 6, or continuously received by the light receiving element 6 by way of the apertures 4a. The output of the light receiving element 6 is therefore either a zero or a constant level direct current output. Hence a direct current output is not produced by the rectifying circuit 8.
Accordingly, when the motor 1 is rotating the output from an inverter 9 becomes zero (an output of logic value zero), while when the motor 1 is not rotating an output of a predetermined level (output of logic value 1) is generated from the inverter 9. This enables a construction wherein the solenoid of the door lock is excited by the output from the inverter 9, so that when the motor 1 is stopped, the solenoid of the door lock is excited and the lock released.
However, with the conventional sensor such as described above, for detecting the stopping of the mechanical moving part, when for example a fault occurs in the light emitting element 5, the light receiving element 6, the AC amplifier 7 or the rectifying circuit 8, such that an alternating current signal based on the light beam is not transmitted, then if the inverter 9 is operating normally, the output of the inverter remains at a logic value of 1 irrespective of operation of the motor 1, so that an output signal for release of the door lock is produced.
This is because, with such a conventional sensor, the stopping of the mechanical moving part is indicated by an output of a low energy condition, and this output signal is subjected to NOT operation (by the inverter 9) to, thereby, produce an output signal of logic value 1 corresponding to the high energy condition for solenoid excitation. With this sensor construction which includes the NOT operation circuit, then even with a fail safe construction wherein an output signal of logic value 1 is not produced at the time of a fault in the NOT operation circuit, if a disconnection fault occurs in the input signal supply line to the NOT operation circuit, then the erroneous generation of an output signal of logic value 1 will occur. Such a disconnection fault in the input signal line has the same effect as the occurrence of a fault in the AC amplifier 7, the rectifying circuit 8, or the light emitting/light receiving elements 5, 6 in FIG. 1.
Accordingly, an interlock system using a sensor which errs in this way to a logic value of 1, cannot be considered sufficient for guaranteeing the safety of the operator.
The present invention takes into consideration the abovementioned situation with the object of providing a safety ensuring apparatus which can reliably ensure the safety of the operator, by utilizing a sensor which directly indicates the stopping of the mechanical moving part with an output signal of logic value 1 corresponding to a high energy condition. A further object of the present invention is to provide a safety ensuring apparatus which can reliably ensure the safety of the operator, by utilizing a fail safe on-delay circuit. Moreover, the safety ensuring apparatus of the present invention is able to provide an interlock system which meets the functional requirements for "Solenoid Locking Safety Switches" covered by British Standard BS5304, Para. 9.7.2.3.2.